A spa (also commonly known as a xe2x80x9chot tubxe2x80x9d when located outdoors) is a therapeutic bath in which all or part of the body is exposed to forceful whirling currents of hot water. When located indoors and equipped with fill and drain features like a bathtub, the spa is typically referred to as a xe2x80x9cwhirlpool bathxe2x80x9d. Typically, the spa""s hot water is generated when water contacts a heating element in a water circulating heating pipe system. A major problem associated with the spa""s water circulating heating pipe system is the risk of damage to the heater and adjacent parts of the spa when the heater becomes too hot.
FIG. 1 is a drawing showing the main elements of a prior art hot tub spa system 1. Spa controller 7 is programmed to control the spa""s water pumps 1A and 1B and air blower 4. In normal operation, water is pumped by water pump 1A through heater 3 where it is heated by heating element 5. The heated water then leaves heater 3 and enters spa tub 2 through jets 11. Water leaves spa tub 2 through drains 13 and the cycle is repeated.
Some conditions may cause little or no flow of water through the pipe containing heating element 5 during the heating process. These problems can cause what is known in the spa industry as a xe2x80x9cdry firexe2x80x9d. Dry fires occur when there is no water in heater 3 or when the flow of water is too weak to remove enough heat from the heating element 5. Common causes of low water flow are a dirty filter or a clogged pipe. For example, referring to FIG. 1, if a bathing suit became lodged in pipe 17B clogging the pipe, flow of water through heater 3 would be impeded and a dry fire could occur.
FIG. 1 shows a prior art arrangement to prevent overheating conditions. A circuit incorporating temperature sensor 50 serves to protect spa 1 from overheating. Temperature sensor 50 is mounted to the outside of heater 3. Temperature sensor 50 is electrically connected to comparator circuit 51A and control circuit 52A, which is electrically connected to high limit relay 53A.
As shown in FIG. 1, power plug 54 connects heating element 5 to a suitable power source, such as a standard household electric circuit. Water inside heater 3 is heated by heating element 5. Due to thermal conductivity the outside of heater 3 becomes hotter as water inside heater 3 is heated by heating element 5 so that the outside surface of heater 3 is approximately equal to the temperature of the water inside heater 3. This outside surface temperature is monitored by temperature sensor 50. Temperature sensor 50 sends an electric signal to comparator circuit 51A corresponding to the temperature it senses. When an upper end limit temperature limit is reached, such as about 120 degrees Fahrenheit, positive voltage is removed from the high temperature limit relay 53A, and power to heating element 5 is interrupted.
A detailed view of comparator circuit 51A and control circuit 52A is shown in FIG. 4. Temperature sensor 50 provides a signal representing the temperature at the surface of heater 3 to one input terminal of comparator 60. The other input terminal of comparator 60 receives a reference signal adjusted to correspond with a selected high temperature limit for the surface of heater 3. As long as the actual temperature of the surface of heater 3 is less than the high temperature limit, comparator 60 produces a positive or higher output signal that is inverted by inverter 62 to a low or negative signal. The inverter output is coupled in parallel to the base of NPN transistor switch 64, and through a normally open high limit reset switch 66 to the base of a PNP transistor switch 68. The low signal input to NPN transistor switch 64 is insufficient to place that switch in an xe2x80x9conxe2x80x9d state, such that electrical power is not coupled to a first coil 70 of a twin-coil latching relay 74. As a result, the switch arm 76 of the latching relay 74 couples a positive voltage to control circuit 52A output line 78 which maintains high limit relay 53A in a closed position (FIG. 1).
As shown in FIG. 4, in the event the switch arm 76 of the latching relay 74 is not already in a position coupling the positive voltage to the output line 78, momentary depression of the high limit reset switch 66 couples the low signal to the base of PNP transistor switch 68, resulting in energization of a second coil 72 to draw the switch arm 76 to the normal power-on position.
If the water temperature increases to a level exceeding the preset upper limit, then the output of the comparator 60 is a negative signal which, after inversion by the inverter 62, becomes a high signal connected to the base of NPN transistor switch 64. This high signal switches NPN transistor switch 64 to an xe2x80x9conxe2x80x9d state, and thus energizes the first coil 70 of latching relay 74 for purposes of moving the relay switch arm 76 to a power-off position. Thus, the positive voltage is removed from the high temperature limit relay 53A, and power to heating element 5 is interrupted. Subsequent depression of the high limit reset switch 66 for resumed system operation is effective to return switch arm 76 to the power-on position only if the temperature at the surface of heater 3 has fallen to a level below the upper limit setting.
In addition to the circuit incorporating temperature sensor 50, it is an Underwriters Laboratory (UL) requirement that there be a separate sensor located inside heater 3 in order to prevent dry fire conditions. There are currently two major types of sensors that are mounted inside of heater 3: water pressure sensors and water flow sensors.
FIG. 1 shows water pressure sensor 15 mounted outside heater 3. As shown in FIG. 1, water pressure sensor 15 is located in a circuit separate from temperature sensor 50. It is electrically connected to spa controller 7, which is electrically connected to regulation relay 111.
Spa controller 7 also receives an input from tub temperature sensor 112. A user of spa 1 can set the desired temperature of the water inside tub 2 to a predetermined level from keypad 200. When the temperature of the water inside tub 2 reaches the predetermined level, spa controller 7 is programmed to remove the voltage to regulation relay 111, and power to heating element 5 will be interrupted.
In normal operation, when water pressure sensor 15 reaches a specific level, the electromechanical switch of the sensor changes its state. This new switch state indicates that the water pressure inside heater 3 is large enough to permit the heating process without the risk of dry fire. Likewise, in a fashion similar to that described for temperature sensor 50, when a lower end limit pressure limit is reached, such as about 1.5-2.0 psi, positive voltage is removed from regulation relay 111, and power to heating element 5 is interrupted.
However, there are major problems associated with water pressure sensors. For example, due to rust corrosion, these devices frequently experience obstruction of their switch mechanism either in the closed or open state. Another problem is related to the poor accuracy and the time drift of the pressure sensor adjustment mechanism. Also, water pressure sensors may have leaking diaphragms, which can lead to sensor failure. The above problems inevitably add to the overall expense of the system because they may require relatively frequent replacement and/or calibration of water pressure sensor switch.
Another known solution to the dry fire problem is the installation of a water flow sensor 16 into the heating pipe, as shown in FIG. 2. However, like the water pressure sensor, water flow sensor 16 is prone to mechanical failure in either the open or close state. Moreover, water flow sensor switches are expensive (approximately $12 per switch) and relatively difficult to mount.
It is known in the prior art that it is possible to substitute a microprocessor in place of the comparator circuit and control circuit, as shown in FIG. 3. Microprocessor 56A is programmed to serve the same function as comparator circuit 51A and control circuit 52A (FIG. 1). When an upper end limit temperature limit is reached, such as about 120 degrees Fahrenheit, microprocessor 56A is programmed to cause positive voltage to be removed from high temperature limit relay 53A, and power to heating element 5 is interrupted.
Resistive water level sensors (also known as resistive fluid level sensors) are known. A resistive water level sensor functions by utilizing a probe to sense the presence or absence of water in a water container. FIGS. 8A and 8B illustrate the operation of a resistive water level sensor. FIG. 8B shows water 204 in container 203. Electrically conductive probe 201 is held in place inside container 203 by insulating sleeve 200. A conductive wire extends from the top of probe 201 to electronic circuit 206. Conductor 202 is mounted to the side of container 203 and is grounded. As shown in FIG. 8B, the water level is below probe 201. Therefore the resistance between probe 201 and conductor 202 is substantially infinite. Hence, no current would flow through the electronic circuit. In FIG. 8A, the water level has increased so that it is above the tip of probe 201. The resistance through water 204 is relatively low and a current carrying path is established between probe 201 and conductor 202, completing the electronic circuit.
A popular application of resistive water level sensors is their utilization to sense to presence or absence of boiler water in heating plant boilers. Advantages of resistive water level sensors are that they have a relatively simple design, requiring low maintenance and are relatively inexpensive.
What is needed is a better device for preventing dry fire conditions in a hot tub spa.
The present invention provides a dry fire protection system for a spa and the spa""s associated equipment. A heating element heats the spa""s water. A resistive water level sensor senses that the level of water around the heating element is higher than a predetermined height or lower than a predetermined height, and a heating element deactivation device electrically deactivates the heating element when the water level around the heating element falls below a predetermined level. In a preferred embodiment, the heating element deactivation device is an electric circuit comprising a comparator circuit and a control circuit.